1. Field of the Invention
The invention relates to a voice signal coder including an improved voice activity detector, and in particular a coder conforming to ITU-T Standard G.729A, Annex B.
2. Description of the Prior Art
A voice signal contains up to 60% silence or background noise. To reduce the quantity of information to be transmitted, it is known in the art to discriminate between voice signal portions that really contain wanted signals and portions that contain only silence or noise, and to code them using respective different algorithms, each portion that contains only silence or noise being coded with very little information, representing the characteristics of the background noise. This kind of coder includes a voice activity detector that effects the discrimination in accordance with the spectral characteristics and the energy of the voice signal to be coded (calculated for each signal frame).
The voice signal is divided into digital frames corresponding to a duration of 10 ms, for example. For each frame, a set of parameters is extracted from the signal. The main parameters are autocorrelation coefficients. A set of linear prediction coding coefficients and a set of frequency parameters are then deduced from the autocorrelation coefficients. One step of the method of discriminating between voice signal portions that really contain wanted signals and portions that contain only silence or noise compares the energy of a frame of the signal with a threshold. A device for calculating the value of the threshold adapts the value of the threshold as a function of variations in the noise. The noise affecting the voice signal comprises electrical noise and background noise. The background noise can increase or decrease significantly during a call.
Also, noise frequency filtering coefficients must also be adapted to suit the variations in the noise.
The paper “ITU-T Recommendation G729 Annex B: A Silence Compression Scheme for Use With G729 Optimized for V.70 Digital Simultaneous Voice and Data Applications”, by Adil Benyassine et al., IEEE Communication Magazine, September 1997, describes a coder of the above kind.
The decoder which decodes the coded voice signal must use alternately two decoder algorithms respectively corresponding to signal portions coded as voice and signal portions coded as silence or background noise. The change from one algorithm to the other is synchronized by the information coding the periods of silence or noise.
Prior art codes that implement ITU-T Standard G.729A, Annex B, 11/96, are no longer capable of distinguishing between a wanted signal and noise if the noise level exceeds 8 000 steps on the quantization scale defined by the standard. This results in many unnecessary transitions in the voice activity detection signal and thus in the loss of wanted signal portions.
A prior art solution described in contribution G.723.1 VAD consists of totally inhibiting voice activity detection in the coder when the signal-to-noise ratio is below a predetermined value. This solution preserves the integrity of the wanted signal but has the drawback of increasing the traffic.
The object of the invention is to propose a more efficient solution, which preserves the efficiency of voice activity detection in terms of traffic, but which does not degrade the quality of the signal reproduced after decoding.